Transcript A Big Test Result

Introduction to Computer
Networks
How Networks Send Data
This lesson introduces the concept of packets as
the basic building blocks of network data
communications.
The Function of Packets
• Data usually exists as rather large files. However,
networks cannot operate if computers put large
amounts of data on the cable at the same time.
• Large
continuous
streams of
data slow
down the
network.
The Function of Packets
• In order for many users at once to transmit data
quickly and easily across the network, the data
must be broken into small, manageable chunks.
• This way, users each get their share of access to
the network. These chunks are called packets, or
frames.
• When the network
operating system at the
sending computer breaks
the data into packets, it
adds special control
information to each
frame.
Packet Structure
• Header: 1. An alert signal to indicate that the
packet is being transmitted. 2. The source address.
3. The destination address. 4. Clock information
to synchronize transmission.
• Data: From 0.5 kilobytes (KB) to 4 KB.
• Trailer : contains an errorchecking component called
a cyclical redundancy check
(CRC). The CRC is a
number produced by a
mathematical calculation on
the packet at its source.
Example: Packets in Printing (1)
• A large print job must be sent from a computer to
a print server.
• 1. The sending computer establishes a connection
with the print server.
Example: Packets in Printing (2)
• 2. The sending computer breaks the large print
job into packets.
• Each packet contains the destination address, the
source address, the data, and control information.
Example: Packets in Printing (3)
• 3. The network interface card (NIC) in each
computer examines the receiver's address on all
frames sent on its segment of the network.
• until it
detects a
frame
addressed
specifically
to it.
Example: Packets in Printing (4)
• 4. The destination computer is the print server.
The packets enter through the cable into the NIC.
• 5. The
network
software
processes
the frame
stored in
the NIC's
receive
buffer.
Example: Packets in Printing (5)
• 6. the network operating system in the receiving
computer reassembles the packets back into the
original file and moves the file into the computer's
memory.
• From
there the
file is sent
to the
printer.